The present disclosure relates generally to a system and method to monitor the isolation resistance in a fuel cell system and more particularly to estimate the coolant conductivity circulating in the fuel cell system.
Current fuel cell technology requires a low conductivity (high resistance) coolant to prevent leakage current from flowing between the stack in the remainder of the system. Leakage current flowing through the coolant can cause short circuiting, induce galvanic corrosion and electrolyze the coolant, reducing engine efficiency. Generally non-corrosive coolants such as water, antifreeze, or mixtures thereof, etc., are used in the bipolar plates. Over time, however, the internal heat exchange faces of the bipolar plates begin to dissolve. As the coolant ages it collects contaminants that cause it to become electrically conductive such that the stack coolant could conduct a leakage current throughout the coolant loop.
Electrical isolation resistance monitoring is required for many fuel cell automotive systems. Typically, the isolation resistance is monitored on the propulsion high voltage bus. If the fuel cell stack is interfaced to the propulsion bus through a voltage converter, its isolation resistance is scaled by some function of the converter's voltage gain. The exact function is dependent on the converter type and which high voltage rail, positive or negative, is common to the system.
A general isolation fault requires that a service technician isolate components from the high voltage bus one-by-one to isolate if the root cause of the isolation fault is the coolant. This is undesirable because of the time and labor needed to track down the cause of the fault. Accordingly, a need exists for alternative methods to determine if coolant conductivity is excessively high and the cause of the fault.